Well, following the comment I added to my last blog post, Ferrari have announced that Michael Schumacher is indeed going to stand in for Massa at the European Grand Prix, and for as long as Massa is out. Schumacher hasn't raced for a couple of years but has kept training. It'll be interesting to see how competitive he is, and also whether Kimi Raikkonen ups his game with his former rival as a team-mate.
At 40 years old, Michael doesn't have youth on his side any more, but there are plenty of drivers that have raced in F1 beyond that age - although not many in the modern era. Incidentally, Damon Hill stopped at 39.